clang-format: Explicitly use python3

[arsenm]

Summary

On Ubuntu 20.04, /usr/bin/env python always fails and requires
explicitly choosing python2 or python3.

Grep shows there are a lot of other places still relying on the old,
sensible behavior but these are the annoying ones I keep running into.

[compnerd]

While I really like this idea of migrating the scripts to python3, I believe that the current plan is to allow until December for users to migrate, so this might be a bit premature :-(.

[arsenm]

In D82767#2123024, @compnerd wrote:

While I really like this idea of migrating the scripts to python3, I believe that the current plan is to allow until December for users to migrate, so this might be a bit premature :-(.

Is this documented somewhere? I think that every script being broken by default on all new distributions is a big problem. It's been 12 years, and python 2 has been EOL since January. I don't see why downstream projects would still be holding on dropping python 2

[compnerd]

The thread that I was basing the timeline from is http://lists.llvm.org/pipermail/llvm-dev/2020-January/138730.html.

[MyDeveloperDay]

We may not be consistent across all of LLVM

$ find . -name '*.py' -print -exec /usr/bin/head -2 {} \; | grep "#!" | sort | uniq -c
      6 #! /usr/bin/env python
      2 #! /usr/bin/env python3
      2 #! /usr/bin/python
      1 #!/bin/env python
    133 #!/usr/bin/env python
     13 #!/usr/bin/env python3
     49 #!/usr/bin/python

[compnerd]

Thinking a bit more about this, using /usr/bin/env python ensures that we actually honour the python version that the user specifies because that allows the user control over the python symlink pointing to either python2 or python3. @arsen what do you think of just unifying on /usr/bin/env python across all the scripts?

[serge-sans-paille]

In D82767#2132903, @MyDeveloperDay wrote:

We may not be consistent across all of LLVM

$ find . -name '*.py' -print -exec /usr/bin/head -2 {} \; | grep "#!" | sort | uniq -c
      6 #! /usr/bin/env python
      2 #! /usr/bin/env python3
      2 #! /usr/bin/python
      1 #!/bin/env python
    133 #!/usr/bin/env python
     13 #!/usr/bin/env python3
     49 #!/usr/bin/python

My understanding is that explicitly requiring python3 may make sense if the script is not backward-compatible with python2, while requiring python means the version is not important.
At the end-of-year, we should be able to harmonize shebangs to #!/usr/bin/env python3 or #!/usr/bin/env python

[arsenm]

In D82767#2141962, @serge-sans-paille wrote:

In D82767#2132903, @MyDeveloperDay wrote:

We may not be consistent across all of LLVM

$ find . -name '*.py' -print -exec /usr/bin/head -2 {} \; | grep "#!" | sort | uniq -c
      6 #! /usr/bin/env python
      2 #! /usr/bin/env python3
      2 #! /usr/bin/python
      1 #!/bin/env python
    133 #!/usr/bin/env python
     13 #!/usr/bin/env python3
     49 #!/usr/bin/python

My understanding is that explicitly requiring python3 may make sense if the script is not backward-compatible with python2, while requiring python means the version is not important.
At the end-of-year, we should be able to harmonize shebangs to #!/usr/bin/env python3 or #!/usr/bin/env python

Fixing this to be consistent would be an improvement

[serge-sans-paille]

My understanding is that explicitly requiring python3 may make sense if the script is not backward-compatible with python2, while requiring python means the version is not important.
At the end-of-year, we should be able to harmonize shebangs to #!/usr/bin/env python3 or #!/usr/bin/env python

Fixing this to be consistent would be an improvement

Change proposed in https://reviews.llvm.org/D83857

clang/tools/clang-format/clang-format-diff.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 #===- clang-format-diff.py - ClangFormat Diff Reformatter ----*- python -*--===#
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===------------------------------------------------------------------------===#

clang/tools/clang-format/git-clang-format

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 #===- git-clang-format - ClangFormat Git Integration ---------*- python -*--===#
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===------------------------------------------------------------------------===#
 
 r"""
 clang-format git integration
 ============================
 
 This file provides a clang-format integration for git. Put it somewhere in your
 path and ensure that it is executable. Then, "git clang-format" will invoke
 clang-format on the changes in current files or a specific commit.
 
 For further details, run:
 git clang-format -h
 
-Requires Python 2.7 or Python 3
+Requires Python 3
 """
 
 from __future__ import absolute_import, division, print_function
 import argparse
 import collections
 import contextlib
 import errno
 import os